Practical: Investigating Respiration (Edexcel IGCSE Biology): Revision Note
Exam code: 4BI1
Practical: Investigating Respiration
We can investigate the production of carbon dioxide and heat from respiration through experiments using germinating seeds or other living organisms (eg. blowfly larvae)
Practical investigation: demonstrating the production of carbon dioxide from respiration
Apparatus
Measuring cylinder
3 sets of boiling tubes (two per set)
3 rubber bungs with delivery tubes
Limewater (calcium hydroxide solution)
Damp cotton wool
10 germinating seeds (eg. peas)
10 boiled/dead seeds (control)
10 glass beads (control for volume)
Stand and clamps
Method
Prepare the limewater tubes by pouring 2 cm3 of limewater into the bottom of each of three boiling tubes
Prepare the seed tubes by adding damp cotton wool to the bottom of three other boiling tubes:
Tube A: 10 germinating seeds
Tube B: 10 boiled/dead seeds
Tube C: 10 glass beads
Seal the seed tubes with a rubber bung and delivery tube, connecting each seed tube to a boiling tube of limewater
Ensure that the end of each delivery tube is submerged a few millimetres below the limewater so that any gas released is bubbled through the limewater
Place the apparatus in a warm environment (around 25-30°C) for several hours/overnight
Observe any colour changes in the limewater
Results
The following results would be expected for this investigation:
Seed tube | Observation | Explanation |
|---|---|---|
A - germinating seeds | Limewater turns cloudy | Germinating seeds respire, releasing carbon dioxide which reacts with limewater, turning it cloudy/milky in colour. |
B - boiled seeds | No change | Boiled seeds will be dead and so will not respire, so no carbon dioxide is released, limewater remains clear. |
C - glass beads | No change | No living material = no respiration = no carbon dioxide produced so limewater remains clear. |
Examiner Tips and Tricks
You could be asked in the exam to give the name of a suitable chemical that could be used as indicator to show that a living organism (eg. germinating seeds, yeast) is respiring . Limewater (calcium hydroxide) changes from a clear solution to cloudy/milky solution when carbon dioxide is bubbled through it.
Other indicators can be used eg. hydrogen carbonate changes colour from orange to yellow when carbon dioxide levels are high.
Practical investigation: demonstrating the production of heat
Apparatus
2 vacuum flasks
2 thermometers
Moist cotton wool
Weak bleach solution
10 germinating seeds (eg. wheat seeds)
10 boiled/dead seeds (killed control, should be same seed type as germinating seeds)
Tweezers
Preparation
Before using the seeds and the vacuum flasks they need to be sterilised carefully with a weak bleach solution (or disinfectant). This is to prevent any respiration from any microbes present causing an increase in temperature, rather than the respiring seeds
Method
Set up the flasks as shown in the diagram below:
Flask A with the dead seeds
Flask B with the germinating seeds
Insert a thermometer into each flask so that the bulb is among the seeds
Seal the top of both flasks with cotton wool
This holds the thermometer in place and also reduces heat loss from the flasks
Invert both flasks and place in an environment where the temperature will remain constant
Record the initial temperature of each flask
After 2-3 days, record the final temperature of each flask
Experiment to demonstrate the production of heat by living material during respiration
Results
The following results would be expected for this investigation:
Flask | Initial temperature (°C) | Final temperature (°C) | Temperature change (°C) | Explanation |
|---|---|---|---|---|
A - boiled dead seeds | 20.0 | 20.0 | 0.0 | Boiled seeds will be dead and so will not respire, so no temperature change would be expected. |
B - germinating seeds | 20.0 | 25.5 | +5.5 | Germinating seeds are respiring, releasing heat energy |
The thermometer in the flask with the germinating seeds (Flask B) should show an increase in temperature, as these seeds will respire which releases heat energy
The seeds in flask A are not respiring because they are dead
If there is a change in temperature in flask A, it is likely due to any change in the room temperature
Therefore an important consideration is to measure the room temperature in addition to the temperature of the flasks
Examiner Tips and Tricks
It's really important that you mention that the seeds (and vacuum flasks) must be sterilised in this experiment to kill any microorganisms present. Otherwise there will be microbial decay during the experiment, which will lead to an increase in the temperature of both flasks.
Applying CORMS
The CORMS prompt can be used to design investigations
The CORMS framework
This is how CORMS could be applied to an investigation demonstrating that carbon dioxide is released by respiring organisms:
Change - change the content of the boiling tube (germinating seeds, dead seeds or glass beads)
Organism - the seeds used should all be of the same type and number/mass
Repeat - repeat the investigation several times to ensure results are reliable
Measurement 1 - observe the change in the limewater indicator
Measurement 2 - ...after 3 hours
Same - control the volume of indicator, the number of seeds/beads, the temperature of the environment
This is how CORMS could be applied to an investigation demonstrating that heat is released by respiring organisms:
Change - change the content of the flasks (germinating seeds or dead seeds)
Organism - the seeds used should all be of the same type and number/mass
Repeat - repeat the investigation several times to ensure our results are reliable
Measurement 1 - observe the change in the temperature on the thermometer
Measurement 2 - ...after 4 days
Same - control the number of seeds, the room/environmental temperature, the material and size of the flasks
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